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+We've now been working on our rewrite of Nix, [Tvix][], for over a
+year.
+
+As you can imagine, this past year has been turbulent, to say the
+least, given the regions where many of us live. As a result we haven't
+had as much time to work on fun things (like open-source software
+projects!) as we'd like.
+
+We've all been fortunate enough to continue making progress, but we
+just haven't had the bandwidth to communicate with you and keep you up
+to speed on what's going on. That's what this blog post is for.
+
+## Nix language evaluator
+
+The most significant progress in the past six months has been on our
+Nix language evaluator. To answer the most important question: yes,
+you can play with it right now – in [Tvixbolt][]!
+
+We got the evaluator into its current state by first listing all the
+problems we were likely to encounter, then solving them independently,
+and finally assembling all those small-scale solutions into a coherent
+whole. As a result, we briefly had an impractically large private
+source tree, which we have since [integrated][] into our monorepo.
+
+This process was much slower than we would have liked, due to code
+review bandwidth... which is to say, we're all volunteers. People have
+lives, bottlenecks happen.
+
+Most of this code was either written or reviewed by [grfn][],
+[sterni][] and [tazjin][] (that's me!).
+
+### How much of eval is working?
+
+*Most of it*! You can enter most (but not *all*, sorry! Not yet,
+anyway.) Nix language expressions in [Tvixbolt][] and observe how they
+are evaluated.
+
+There's a lot of interesting stuff going on under the hood, such as:
+
+* The Tvix compiler can emit warnings and errors without failing
+  early, and retains as much source information as possible. This will
+  enable you to use Tvix as the basis for developer tooling, such as
+  language servers.
+
+* The Tvix compiler performs in-depth scope analysis, so it can both
+  generate efficient bytecode for accessing identifiers, and alert you
+  about problems in your code before runtime.
+
+* The runtime supports tail-call optimisation in many (but – again –
+  not yet all) cases, so you can evaluate recursive expressions in
+  constant stack space.
+
+* The runtime can give you different backing representations for the
+  same Nix type. For example, an attribute set is represented
+  differently depending on whether you've constructed an empty one, a
+  `name/value` pair, or a larger set. This lets us optimise frequent,
+  well-known use-cases without impacting the general case much.
+
+We've run some initial benchmarks against C++ Nix (using the features
+that are ready), and in most cases Tvix evaluation is an order of
+magnitude faster. To be fair, though, these benchmarks are in no way
+indicative of real-life performance for things like `nixpkgs`. More
+information is coming... eventually.
+
+### How does it all work?
+
+Tvix's evaluator uses a custom abstract machine with a Nix-specific
+instruction set, and a compiler that traverses a parsed Nix AST to
+emit this bytecode and perform a set of optimisations and other
+analysis. The most important benefit of this is that we can plan and
+lay out the execution of a program in a way that is better suited to
+an efficient runtime than directly traversing the AST.
+
+TIP: You can see the generated bytecode in [Tvixbolt][]!
+
+This is all written in about 4000 lines of Rust (naturally), some of
+which – especially around scope-handling – are deceptively simple.
+
+As part of our CI suite, we run the evaluator against some tests we
+wrote ourselves, as well as against the upstream Nix test suite (which
+we don't *quite* pass yet. We're working on it!).
+
+### What's next for tvix-eval?
+
+Despite all our progress, there are still some unfinished feature
+areas, and some of them are pretty important:
+
+1. The majority of Nix's builtins – including fundamental ones like
+   `import` and `derivation` – aren't implemented yet.
+
+2. Neither are recursive attribute sets (`rec`). This isn't because of
+   a problem with the recursion itself, but because of the handling of
+   nested keys (such as `a.b`). We have a lackluster solution already,
+   but are designing a more efficient one.
+
+In both cases, we've mostly figured out what to do; now it's just a
+matter of finding the time to do it. Our progress is steady, and can
+be tracked [in the source][src] (viewer without Javascript
+[here][src-noscript]).
+
+Apart from that, the next steps are:
+
+* Comprehensive benchmarking. We're standing up an infrastructure for
+  continuous benchmarking to measure the impact of changes. It'll also
+  let us identify and optimise hotspots
+
+* Implementing known optimisations. There are some areas of the code
+  that have the potential for significant speed gains, but we're
+  holding off implementing those until the evaluator is feature
+  complete and passes the Nix test suite.
+
+* Finishing our language specification. Based on what we've learned,
+  we're writing a specification of the Nix language that captures its
+  various behaviours in all their tricky subtlety and subtle trickery.
+
+Once we can evaluate `nixpkgs`, we're likely to shift our focus
+towards the other areas of Tvix.
+
+## The Other Areas of Tvix
+
+Speaking of these other areas (most importantly, the builder and store
+implementation), we've made some nice progress there also.
+
+While we've yet to start assembling the actual pieces, [flokli][] and
+[adisbladis][] have been hard at work on [go-nix][], which aims to
+implement many of the low-level primitives required for the Nix store
+and builder (hashing and encoding schemes, archive formats, reference
+scanning ...).
+
+We're looking forward to telling you more in the next Tvix status
+update!
+
+## Outro ...
+
+We'd be delighted to onboard new contributors to Tvix! Please take a
+look at the main [TVL page](https://tvl.fyi) to find out how to get in
+touch with us if you'd like to join!
+
+Thanks also, of course, to [NLNet](https://nlnet.nl/) for sponsoring
+some of this work!
+
+And finally, we would like to thank and pay our respects to jD91mZM2 –
+the original author of
+[rnix-parser](https://github.com/nix-community/rnix-parser) – who has
+sadly passed away. Please, tell people how important they are to you.
+
+We use `rnix-parser` in our compiler, and its well-designed internals
+(also thanks to its new maintainers!) have saved us a lot of time.
+
+That's it for this update. Go play with [Tvixbolt][], have fun
+figuring out weird ways to break it – and if you do, let us know.
+
+We'll see you around!
+
+[Tvix]: https://tvl.fyi/blog/rewriting-nix
+[Tvixbolt]: https://tvixbolt.tvl.su
+[integrated]: https://cl.tvl.fyi/q/status:merged+%2522tvix/eval%2522+mergedbefore:2022-09-09
+[src]: https://cs.tvl.fyi/depot/-/tree/tvix/eval
+[src-noscript]: https://code.tvl.fyi/tree/tvix/eval
+[tazjin]: https://tazj.in
+[grfn]: https://gws.fyi/
+[sterni]: https://github.com/sternenseemann
+[go-nix]: https://github.com/nix-community/go-nix
+[flokli]: https://flokli.de/
+[adisbladis]: https://github.com/adisbladis